2.1b: The umpires shall have discretion, for other wholly acceptable reasons, to allow a substitute for a fielder, or a runner for a batsman, at the start of the match or at any subsequent time

2.2: The opposing captain shall have no right of objection to any player acting as a substitute on the field, nor as to where the substitute shall field. However, no substitute shall act as wicket-keeper

England have not broken any rules in using agile fielding specialists to
replace tired fast bowlers during the Ashes series.

But Australia clearly feel that the spirit of the game has been breached by
the home side in their use of "12th men".